The other day I was reading about ring signatures. I don’t know if they work for Nostr private keys, but if they do, we can use two private keys—the sender’s and a random one—in a ring signature. Then we can reveal an arbitrary part of the random private key in the “ephemeral message” so we can control how difficult it is to guess it. 

 The thing with time-stamping is you can't prevent someone else from time-stamping your data. If someone can prove that the signature existed prior to when someone else could have cracked the key, that's evidence that the signature wasn't faked.